Heat flux meters have previously been described by Northover et al in J. Sci. Instrum., 1967, Vol. 44, pp 371-374. As described in that article, a heat flux meter intended for use in furnaces of steam generators comprises a cylindrical body member and a disc member joined to the cylindrical body. The assembly is mounted to a boiler wall, typically by welding, thereby forming an internal cavity.
The principle of operation of this heat flux meter is that heat flowing into the disc distributes radially to the cylindrical body and is then conducted to the boiler wall. The finite thermal resistance of the disc gives rise to a temperature difference between its centre and periphery. Upon attachment of suitable connecting wires to the disc centre and the cylindrical body, an e.m.f. is produced which is proportional to the magnitude of the disc radial temperature difference and, therefore, the heat flux. The connecting wires are constructed of the same material as the cylindrical body while the disc material is thermoelectrically-dissimilar to the cylindrical body material.
A connector tube of the same material as the meter body is joined to it by welding. When the flux meter is mounted in a boiler, both the body and the connector tube are welded to a boiler tube to provide a low resistance heat path through all parts of the meter system. The connecting tube is made long enough to protrude outside the boiler into a region at or near room temperature. The two connecting wires are lead through the connector tube to the interior of the flux meter. A seal is provided between the room temperature end of the connecting tube and the wires, so that a controlled atmosphere may be maintained in the meter cavity.
In this prior art heat flux meter, the disc member is joined to the cylindrical body by a thermoconductive solder, which is specified to be a silver-based solder in the Northover et al disclosure. An oxygen-free atmosphere must be maintained within the heat flux meter cavity to avoid destruction by oxidation of the various joints within the meter, in particular between the wires and the disc member and/or the body member. Heat flux meters are required to have a service life of, preferably, several years and over such extended periods of time, even the very slow ingress of corrosive elements may prove destructive.
It has now been found that these prior art heat flux meters are prone to failure upon extended periods of operation, especially when mounted in those regions of the boiler which experience the highest heat fluxes, namely the furnace walls in the vicinity of the burners. Examination of failed flux meters reveals corrosive attack at the joint between disc and body members and corrosive oxidative failure of the joint between the disc and the connecting wire. In the high heat flux service applications, the temperature of the disc centre rises several hundred degrees Celsius above the temperature of the tube walls. The disc may then be required to operate above the melting temperature of commercial silver-based solders.
Any junction between two dissimilar metals is known to be an area of enhanced corrosive attack. The presence of any solder layer, but in particular a silver-based solder at the interface of the disc and the body members, amplifies the effect. Silver, and liquid silver even more so than solid silver, has a high solubility for oxygen and sulfur, the two principal corrosive elements in the boiler atmosphere. As a result of this property, the silver solder layer acts as a preferred path for the diffusion of the corrosive atoms to reaction sites, so as to enhance the rate of corrosive attack.
While the applicant does not wish to be bound by any theory to explain the observed failures of the prior art heat flux meters, they are thought to occur by oxidation and destruction of the joint between the disc and body members as a result of corrosion at the high temperatures typically encountered in boilers. The concomitant ingress of oxygen and possibly sulfur into the flux meter cavity of the flux meter leads to corrosive attack upon joint between connecting wire and disc and ultimately destruction of the meter. Moreover, before destruction, corrosion at the disc/body solder joint increases the thermal resistance of the joint, resulting in an unstable heat flux versus e.m.f. calibration.